There are many types of particle counters. This invention evolved from particle counters of the type commonly known as a Coulter counter. U.S. Pat. No. 2,869,078 describes a counter in which a suspension of particles to be counted flows from an upstream vessel through an aperture to a downstream vessel. An electrical current is established through the aperture by electrodes placed in each of the vessels. The displacement of fluid by a particle in the sensing zone of the sensing aperture causes a change in impedance in this passage. That change in impedance causes a change in current or in voltage which is detected by a suitable detector. The particle laden fluid is caused to flow through the aperture into the downstream vessel by application of a differential pressure such as by withdrawing liquid from the fluid-filled and otherwise closed downstream vessel. The volume of liquid which leaves the closed downstream vessel through an outflow port equals exactly to the volume of liquid traveling through the sensing aperture. By counting the particles as a predetermined volume of fluid leaves the closed downstream vessel, one can obtain the concentration of particles in the particle laden fluid in the upstream vessel.
In the early Coulter particle counters, the downstream vessel quickly became contaminated by analyzed waste and there were no attempts to let only clean fluid reach the sensitive region at the downstream side of the sensing aperture. As a result extraneous particles traveled into that sensing zone and gave rise to extraneous signals and to a buildup of deleterious precipitates. U.S. Pat. No. 3,299,354 provided an additional downstream chamber with an elongated snout having a large capture orifice placed opposite the much smaller sensing aperture. The thin fluid jet formed by the flow of particle laden fluid through the very small sensing aperture was directed into the large capture orifice. The intent was to trap all of the suspended and dissolved particulates. However, according to later patent applications assigned to the assignee of that patent, because of eddy currents and other irregularities, all particulates were not captured by the snout, extraneous counts were still generated at the sensing orifice, and it proved essential to use a burn circuit to keep the rear of the sensing orifice free of particulate precipitates. U.S. Pat. No. 3,746,976 describes an apparatus employing two downstream chambers with an elongated capture snout opposite the sensing aperture. A pump disposed between the two downstream chambers recirculated fluid through a filter to the sensitive zone at the downstream side of the sensing aperture. The action of the pump was to provide clean fluid to hydrodynamically focus the thin particle laden jet from the sensing aperture so that it passed through the first downstream chamber and on through the capture orifice without any of the sample suspension remaining in the region of the sensing aperture, or being able to return to that region. U.S. Pat. No. 4,360,803 is directed to a similar particle analyzing apparatus in which the fluid is allegedly pumped only by the kinetic energy of the thin jet from the sensing aperture and this jet is alleged to be hydrodynamically focused through the large capture orifice. Once one computes the fluidic resistances and pressure gradients in the recirculation circuits and across the capture orifices of any of the structures taught in '803 there is certainly insufficient kinetic energy in the thin jet from the sensing aperture to accelerate the entire merry-go-round cycle through the large capture orifice to beyond the velocity of the incoming jet. Hence, compared with dimensions and velocities of the sample stream jet at the exit of the sensing aperture, there can be no net sample stream acceleration or diameter contraction at the passively functioning cleaning orifice; hence no hydrodynamic focusing. Indeed, despite entrainment of much clean fluid by the thin fluid jet from the small sensing aperture, the resulting expanding, fluttering, ensheathed sample jet is still so small relative to the large capture orifice that it promotes intermittent counter current streams of contaminated fluid to periodically emerge from the capture snout to the sensitive region of the sensing aperture.
The pressure built up in an expansion chamber to create a fluid flow from that chamber is the classic head (static) pressure which also behaves as a backpressure at the entrance to the expansion chamber. This backpressure is well documented in the literature on combustion engines and fan efficiency. In axial fans, it can cause the fluid near the hub to move in the wrong direction through the fan. In the cited particle capture arrangements the head pressure forces waste fluid to flow down the fluctuating pressure field to periodically jet waste contaminants right back out of the "cleaning orifice" and right into the rheologic sump adjacent to the jet emerging from the sensing aperture and within the internal electric fringe field of the "closely spaced" sensing aperture.
U.S. Pat. No. 4,710,021 describes a particulate matter analyzing apparatus in which the original downstream vessel of the Coulter counter is a closed chamber partitioned into two regions, proximal and distal, by a barrier which includes a large capture orifice directly opposite the small sensing aperture formed in the wall of the chamber. The thin sample jet emerging from the small sensing aperture travels through the proximal region of the divided chamber, on through the large capture orifice, and strikes the opposite (distal) chamber wall where it was believed that the kinetic energy was dissipated. However, detailed studies have now shown that as a result of the Coanda surface flow effect, particulate matter rapidly travels along the walls of the distal chamber regions, all faces of the barrier, and the walls of the proximal chamber. Thus particulates and solutes rapidly reach the sensing aperture where they generate extraneous signals and soon disperse throughout the proximal region of the partitioned chamber.
Apparatus exploiting this invention for better resistivity leukocyte analysis has also had to resort to a so-called burn circuit to keep particulate debris from building up in the sensitive region on the downstream side of the sensing aperture. Additionally, Coanda effect contamination throughout the proximal region of the divided chamber has been too high to permit the envisioned exploitation of the region between the sensing aperture and the capture orifice for optical interrogations of the particles or solutes in the thin sample jet.
In addition, because of the necessarily large size of the barrier orifice, there is a tendency for regurgitation of particulates through the orifice via well known countercurrents. Even with the best manufacturing tolerances, a precise axial alignment of the sensing aperture and cleaning orifice can not always be achieved resulting in only a fluttering portion of the sample jet being captured if the capture orifice is made smaller. On the other hand, if the capture orifice is enlarged, this leaves more area for direct regurgitation of particulates driven by the kinetically unopposed head pressure explained above.